The traditional method for evaluating protein quality has been the protein efficiency ratio (PER) which has been in use since 1919. PER is a measure of the weight gain of a growing animal divided by the amount of protein that the animal consumes.

The PER is based on the growth of rats, in response to a given amount of protein. However, rats have different amino acids requirements than we do. In particular, their replacement for methionine is thought to be 50 percent higher. Consequently, the PER undervalues the quality of legume proteins.

Recently, the Protein Digestibility Corrected Amino Acid Score (PDCAAS) was introduced as a more accurate way to evaluate protein quality for children and adults. The PDCAAS has now been adopted as the official method by the World Health Organization, the U.S. Food and Drug Administration and the U.S. Department of Agriculture.

The PDCAAS is the amino acid score (amino acid pattern of a protein relative to amino acid needs) with a correction factor for digestibility. The PDCAAS uses human, not rat, amino acid requirements to calculate the amino acid score. Specifically, the amino acid requirements used are those established by the World Health Organization (WHO) for two- to five-year-old children. Those proteins that meet the WHO guidelines are a given a rating of 1.

Soy protein contains enough of all the essential amino acids to meet biological requirements when consumed at the recommended level of protein intake.

The PDCAAS for soy protein isolate is 1.0, which makes soy equivalent to animal proteins in quality and higher than other plant proteins.

On average, Americans eat too much protein. The current recommendation is that protein makes up around 15-20 percent of your total calories for the day. Protein has four calories per gram, so for a 2000 calorie-a-day diet you could eat 75-100 grams of protein. However, most of us consume more than that each day. Eating too much protein can be bad for your body.

Growing children need about one gram of protein for every two pounds of body weight. For example, a 56 pound child needs about 28 grams of protein a day to sustain her growth.

Adapted from The Doctor's Pocket Calorie, Fat and Carbohydrate Counter. For more information, see the book or the CalorieKing.com online food database.